Summary/Abstract An in-depth understanding of transmission of malaria from the human host to the mosquito vector is required to develop and target improved interventions to further reduce the burden of malaria to its ultimate elimination. However, our comprehension of what governs natural malaria transmission is far from complete. We know that malaria transmission depends on the presence of mature gametocytes in the peripheral blood, yet the rate at which gametocytes are produced, and factors associated with gametocyte commitment, maturation and infectivity, remain poorly elucidated. Few studies have accurately quantified the likelihood of transmission from the infected host to the vector, and fewer still the contribution of different age groups within populations to overall transmission. In high transmission settings, sub-microscopic infections are an important contribution to the infectious reservoir, and may threaten efforts to control and eliminate malaria. However, data on the infectious reservoir in different settings, and our understanding of the role that microscopically-detectable and sub-microscopic infections play in this reservoir, are limited. In this project, we will utilize blood and mosquito samples collected in different epidemiological settings in Uganda to assess experimental infectivity to mosquitoes, and natural mosquito sampling to address key knowledge gaps, with the following aims: (1) to characterize factors associated with gametocyte production in different transmission intensity settings; (2) to evaluate impacts of human and parasite factors on parasite infectivity to mosquito vectors; and (3) to characterize the human infectious reservoir for malaria. We will test the following hypotheses. (1) Commitment to gametocyte production and maturation is influenced by intrinsic and extrinsic factors, including the immune status of the host, multiplicity of infection, and transmission intensity. (2) Variation in infectivity is explained by intrinsic and extrinsic factors, including host immunity and gametocyte sex ratio, not only gametocyte density. (3) Hosts with asymptomatic malaria infections and school-aged children, who are less well-covered by conventional control measures than other populations, are the most important contributors to the human infectious reservoir. We will utilize the framework of the cohort studies described in the surveillance core together with novel molecular diagnostics to investigate commitment to gametocyte production, gametocyte maturation and infectivity in relation to human host, parasite and environmental characteristics. We will perform repeated assessments of the infectivity of different age groups to locally relevant Anopheles mosquitoes using two feeding approaches and two sources of mosquitoes. Finally, we will quantify the contact between humans and mosquitoes for different age groups over time and space. This project will be tightly integrated with the other two research projects, allowing us to address research questions structured across the parasite life- cycle, and to provide information to help guide strategies for the control and ultimate elimination of malaria in Uganda.